1. Field of the Invention
This invention relates to a stent mounting device and a method of coating a stent using the device.
2. Description of the Background
Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small cavities via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
FIG. 1 illustrates a conventional stent 10 formed from a plurality of rigid but resiliently flexible struts 12 that are arranged in a sinusoidal-like configuration to form a continuous ring or cylinder. The plurality of struts 12 are radially expandable, disposed coaxially, and interconnected by connecting elements 14 that are disposed between adjacent struts 12, leaving lateral openings or gaps 16 between adjacent struts 12. Struts 12 and connecting elements 14 define a tubular stent body having an outer, tissue-contacting surface and an inner surface.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient.
One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
A shortcoming of the above-described method of medicating a stent is the potential for coating defects. While some coating defects can be minimized by adjusting the coating parameters, other defects occur due to the nature of the interface between the stent and the apparatus with which the stent is supported during the coating process. A high degree of surface contact between the stent and the supporting apparatus can provide regions in which the liquid composition can flow, wick, and collect as the composition is applied. As the solvent evaporates, the excess composition hardens to form excess coating on or between the stent struts. Upon the removal of the coated stent from the supporting apparatus, the excess coating may stick to the apparatus, thereby removing some of the needed coating from the stent and leaving bare areas. Alternatively, the excess coating may stick to the stent, thereby leaving excess coating as clumps or pools on the struts or webbing between the struts.
Thus, it is desirable to minimize the interface between the stent and the apparatus supporting the stent during the coating process to minimize coating defects. Accordingly, the present invention provides for a device for supporting a stent during the coating application process. The invention also provides for a method of coating the stent supported by the device.
The present invention provides an apparatus for supporting a hollow, cylindrical stent. The apparatus includes a mounting assembly for releasably holding a stent in a fixed position without being in contact with a majority of the area of the inner surface of the stent. The mounting assembly can be used for supporting a stent during the process of forming a coating on the stent.
In one embodiment, the mounting assembly includes a first coning end that can be inserted within a first end of the stent and a second coning end that can be inserted within an opposing second end of the stent. The coning ends are used for securely pinching the stent.
In one embodiment, the mounting assembly includes a first member for making contact with a first end of the stent, a second member for making contact with a second end of the stent, and a third member for extending within the hollow, cylindrical stent and for securing the first member to the second member. The distance between the first member and the second member can be adjusted by inserting the third member deeper into the first member or the second member.
The mounting assembly can apply a force on a first end of the stent in a first direction and an opposite force on a second end of the stent in a second direction. Accordingly, the mounting assembly prevents any significant movement of the stent.
In one embodiment, the means for applying the forces to the first and second ends of the stent are defined by a pair of members having inwardly tapered ends that penetrate at least partially in the first and second ends of the stent and are in contact with an inner rim of the first and second ends of the stent. In another embodiment, one of the inwardly tapered ends can be incrementally moved closer to the other inwardly tapered end so as to increase the forces applied to the first and second ends of the stent.
The present invention also provides a method of coating a stent. The method includes mounting a stent on a support assembly and forming a coating on the stent. The support assembly does not contact a significant portion of the inner surface of the stent. In one embodiment, the act of mounting includes positioning one end of the stent over a coned end of a first member, positioning a coned end of a second member within the opposing end of the stent, and reducing the distance between the coned ends such that the stent is securely pinched between the coned ends.
In one embodiment, the stent can be at least partially expanded prior to mounting the stent on the support assembly. The coating can be applied by spraying a composition at the stent. Translational and/or rotational motion can be provided during the coating process.